1. Field of the Invention
This invention relates to laser packaging and, in particular, to apparatuses and methods for optically coupling optical fibers to semiconductor lasers.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron lasers, and semiconductor lasers. All lasers have a laser cavity defined by at least two laser cavity mirrors, and an optical gain medium in the laser cavity. The gain medium amplifies electromagnetic radiation (light) in the cavity by the process of stimulated emission.
In semiconductor lasers, a semiconductor active region serves as the gain medium. Semiconductor lasers may be diode (bipolar) lasers or non-diode, unipolar lasers such as quantum cascade (QC) lasers. Semiconductor lasers may also be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, in contrast to SELs, in which the radiation output is perpendicular to the wafer surface, as the name implies. In conventional Fabry-Perot (FP) edge-emitting lasers, a cleaved facet mirror is used to obtain the feedback for laser oscillation. Other semiconductor lasers, such as distributed-feedback (DFB) and distributed-Bragg reflector (DBR) lasers, employ one or more diffraction gratings to provide reflectance.
Semiconductor lasers are used in a variety of applications, such as high-bit-rate optical fiber communications. In telecommunications applications, the laser often emits at a single lasing wavelength at 1.31 xcexcm (and other closely spaced wavelengths), or at telecommunications wavelengths specified by the ITU grid, such as lasing wavelengths of 1.55 xcexcm (and other closely spaced wavelengths). These wavelength ranges are often used for telecommunications purposes because the loss of silica fibers is comparatively low at these wavelengths.
Lasers must be optically coupled to fibers to engage in optical fiber communications. For example, a 1310 nm edge-emitting laser""s output must be optically coupled into the input (light-receiving) end of an optical fiber in order to transmit via the fiber a modulated optical signal generated and output by the laser. It can be difficult, expensive, and time-consuming to properly couple the laser to the fiber. For example, it is difficult to accurately align the laser relative to the fiber or other optical device to which it is to be coupled, so that a sufficient amount of laser light output by the laser is coupled into the fiber. When using a lens which is to be optically interposed between the fiber and laser, the three elements are preferably positioned with respect to each other to achieve sufficient optical coupling.
Active alignment is usually employed to align the fiber with respect to the laser. After the fiber is actively aligned to the laser, the alignment process is completed by mounting its ferrule or other housing to a laser housing such as a TO can housing by a variety of techniques such as laser welding, or by using an appropriate adhesive such as epoxy or glue.
During the alignment process, the lens is either actively aligned, or it is not. If the lens itself is actively aligned with respect to the laser, alignment complexity is increased because the fiber must also be aligned. If the lens is not aligned, the fiber may be actively aligned given whatever position the lens has, but it may not be an optimal combination.
For example, in some conventional techniques, the laser and the input end of the optical fiber to which the laser is to be optically coupled are mounted together in a housing such as a TO (transistor outline) can, along with optics such as a lens disposed between the laser and the fiber end. The fiber end may be disposed in a rigid cylindrical ferrule. Because the lens is between the laser and the fiber, ideally the fiber, lens, and laser are all aligned with respect to each other so that the laser is optically coupled to the fiber. The alignment may therefore involve a first alignment in which the lens is aligned with respect to the laser, and then the fiber is aligned with respect to the already-aligned laser-lens assembly. This requires at least two separate active alignment procedures, adding to alignment complexity.
Alternatively, the lens may simply be placed into fixed position with respect to the laser without actively aligning it, and then aligning the fiber end with respect to the laser-lens assembly. This technique requires only one alignment, but may result in nonoptimal optical coupling if the lens and laser are not properly aligned.